Show a tour when users first create a service

This commit adds a 3 screen tour, similar to those used on GOV.UK Verify
and Passports.

We guerilla tested this on Friday, and it really helped users to build a
mental model of how Notify works, so that when they’re playing around
with it they have a greater sense of what they’re aiming to do. This
makes concepts like templates and placeholders click more quickly.

https://www.pivotaltracker.com/story/show/116710119

app/assets/stylesheets/app.scss

@@ -127,4 +127,4 @@ a[rel="external"] {
   @include media(tablet) {
     @include external-link-19;
   }
-}
+}